BERKELEY, CA – A nanocrystalline
metal is one whose average grain size is measured in billionths of a meter,
much smaller than in most ordinary metals. As the grain size of a metal
shrinks, it can become many times stronger, but it also usually loses
ductility. To take advantage of increasing strength with decreasing grain
size, researchers must first understand a fundamental problem: by what
processes do nanosized crystals of metal stretch, bend, or otherwise deform
under strain?

Frames from a dark-field TEM video
of nanocrystalline nickel under strain show rapid aggregation of a
group of grains.

A team of researchers headed by Scott X. Mao of the Mechanical Engineering
Department of the University of Pittsburgh, working at the National Center
for Electron Microscopy (NCEM) at the Department of Energy's Lawrence
Berkeley National Laboratory, and using high-quality samples of nickel
prepared at DOE's Sandia National Laboratories, has now identified a prominent
way in which nanocrystalline metals deform. The researchers report their
findings in the July 30, 2004 issue of Science.

Ordinary coarse-grained metals deform when parts of a grain slip past
one another as extra planes of atoms, called dislocations, move through
the material. The process has been compared to moving a rug by flapping
one end of it to create a wave, causing the rug to inch along bit by bit.
But the trick won't work if the rug is too short; likewise, if the dimensions
of the crystal grains are too small, dislocations can't be created or
glide through the grain to allow deformation.

Theorists have proposed that when grain sizes are too small for dislocations,
a different mode of deformation comes into play: the grain boundaries
themselves move, sliding past one another and allowing the grains to rotate
to find new ways of fitting together.

"It's a simple idea," says Zhiwei Shan of Mao's laboratory
at Pitt, "and many groups have researched aspects of it, but no one
has reported direct evidence of a shift from dislocation-mediated deformation
to grain-boundary-mediated deformation." Indeed, no one was sure
where to look for the transition from one mode of deformation to the other.
When the grains were reduced to 20 nanometers across? Ten? Perhaps as
small as five?

To search for the effect, Shan used NCEM's In-Situ Microscope, which
he calls "the best in America" for this kind of research. NCEM's
Eric Stach explains that what makes the In-Situ's otherwise standard transmission
electron microscope unique is that it combines a stage area in which samples
can be stressed or manipulated in other ways  and meanwhile videotaped
 with a high voltage, 300-kilovolt electron beam that can penetrate
thick samples and yield excellent 1.9-angstrom resolution.

The nanocrystalline nickel samples were mounted in a probe that placed
them under load  stretched them, in fact  while images of
small regions of the sample were captured on videotape at the standard
rate of 30 frames per second.

But besides having an excellent instrument, says Stach, Zhiwei Shan made
a crucial observation. An effect that was far from obvious in the most
common TEM imaging method, called bright-field imaging, stood out clearly
with the different technique of dark-field imaging.

"As the TEM's electron beam passes through a sample, some of the
electrons are diffracted," Stach explains. "Bright-field images
are constructed using the direct electrons, while dark-field images use
the diffracted electrons. In bright-field imaging, regions of the sample
that scatter a lot of electrons, like defects such as dislocations, look
darker. With dark-field images, strongly diffracting regions look brighter."

Zhiwei Shan of the University of Pittsburgh
works with Eric Stach of Berkeley Lab at the National Center for Electron
Microscopy's In-Situ Microscope.

Shan agrees that "dark-field imaging was critical to the result."
For when he viewed videotapes of the nickel sample under strain, he saw
small regions rapidly brightening and growing larger  direct confirmation
of grains sliding and rotating into positions of strong diffraction.

In a bright-field image these grain-boundary processes would have been
impossible to distinguish from lattice dislocations, which in prior attempts
is what other groups assumed they were seeing. It took dark-field observations
to confirm that below a certain size, grain-boundary rotation indeed becomes
prominent. The cut-off isn't sharp, however.

"It's continuous, not a sharp change," says Shan. "In
describing grain-boundary deformations we chose the word 'prominent' carefully,
because even in nanocrystalline metal, dislocations still play a role."
Trapped dislocations in the crystal lattice were observed even when the
average grain size was as small as 10 nanometers.

Says Stach, "The material always chooses the easiest pathway to
deform, and that can differ through a range of sizes." Although the
In-Situ Microscope observations confirm the grain-boundary model of nanocrystalline
deformation, whichever process predominates at a given grain size depends
on a variety of conditions.

The Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California. Visit our website at http://www.lbl.gov.